I've read about MP3 multiplayer in an IGN interview a few weeks/months ago. They then said they were 'thinking' about it.
I think is really cool how they A. listen to gamers who want multiplayer badly and B. it seems fairly easy to add it. Red Steel is doing it I think and there is a good chance MP3 will have multiplayer too. I was afraid Wii games, especially launch-window games, wouldn't concentrate on multiplayer but I'm glad to see that multiplayer is getting more and more attention. I hope that they will make it all possible through wifi but they may also just be talking about split-screen multiplayer.